This invention relates to fiber reinforced tank structures and to a method of fabricating such tanks.
Filament wound fiberglass tanks have been fabricated in accordance with the prior art by forming the shell of the tank on a full size mandrel in a fabrication shop under controlled conditions and then shipping the tank to the site where it is to be installed. Such an approach has inherent site limitations because of the need for a full size mandrel and for shipping the fabricated tanks. For example, tanks larger than 12 feet in diameter and longer than 50 feet are difficult to transport.
On site fabrication of fiber reinforced resin tanks has also been practiced in the prior art. For example, see U.S. Pat. No. 3,470,656 issued to H. R. Clements. While the problem of transporting a large tank is eliminated, transportation of the men and equipment to fabricate the tank is required. Fabrication of such a tank is a relatively skilled job and typically such skills are not found at the installation site. Further, fabrication of the tank must take place under relatively controlled conditions. For example, the temperature must be above a certain minimum and there cannot be any form of precipitation during fabrication of the tank. Such requirements typically necessitate the building of temporary structures enclosing the location of the tank to be fabricated. The structure of such an enclosure is complicated by the fact that it has to be heated and because of the evolution of large quantities of various resin and solvent vapors during the fiberglass winding operation necessitating extensive ventilation which adds to the difficulty of maintaining a heated enclosure.
As a result, the expenses of on site fabrication include transporting trained men and supervisors to the installation location, feeding and housing them at the installation location and the capital cost of temporary buildings, heating equipment, winding equipment, scaffolding, trucks and tools. Additionally, there is no way to build for stock or schedule the rapid erection of tanks at one site. Further, when multiple tank installations are desired, the tanks may not be able to be placed as closely together as desired because of the need to leave sufficient space between tanks for the operation of the field winding equipment.
The prior art also teaches prefabrication of components of storage tanks, shipping the components to the location of the installation and assembly of the components there. Fabrication of individual panels is taught in U.S. Pat. No. 3,143,306 issued to M. J. Dijkmans et al. However, typical assembly techniques have had substantial disadvantages which are often tolerated because of the need for sufficient circumferential or "hoop" strength in a filled tank. For example, one assembly technique requires supporting the prefabricated panels on a temporary structure and then continuously winding a filament around the entire tank. Such a technique is taught in U.S. Pat. No. 3,843,429 issued to W. B. Jessup. Clearly, many of the disadvantages of complete on site fabrication of the tank are present in such a system. U.S. Pat. No. 2,729,268 issued to D. C. Broughton et al. teaches assembling a cylindrically shaped mandrel on the trailer of a truck and winding on the mandrel a fiber reinforcement impregnated with a resin. After the resin has cured, the mandrel is disassembled and the shell pivoted upwardly from the trailer and tilted into position on a foundation. Clearly, specialized equipment is required and the size of the tank is limited by the size of the equipment to fabricate the entire tank. Another known method of assembling prefabricated sections includes wrapping the outside of a partially completed tank with steel cable. The steel cable is used so the final structure has sufficient strength to withstand the hoop stress developed in a filled tank. The method is disadvantageous because the fiberglass and the steel neither expand nor contract at the same rate upon temperature changes. Further, the cables have little or no resistance to the effects of spilled corrosive liquids. Finally, such an assembly technique produces a finished product with an uneven surface which is not easily cleaned. For example, a structure taught in U.S. Pat. No. 2,074,592 issued to F. F. Rowell used building blocks arranged one upon the other to form staves which are held together by means of tensioned hoops encircling the blocks upon the exterior thereof. These are among the deficiencies of existing storage tank fabrication systems this invention overcomes.